1. Field of the Invention
The present invention relates generally to a gas turbine engine, and more specifically to a variable bypass ratio augmented gas turbine engine used to power an unmanned aero vehicle or UAV.
2. Description of the Related Art including information disclosed under 37 CFR 1.97 and 1.98
An unmanned aero vehicle (UAV) is currently being used for reconnaissance such as for military use. The US Army is a large user of these UAVs because they are small, do not use a lot of fuel, and do not require a pilot on board the aircraft. The main objective of an engine for a UAV is high fuel efficiency at low speeds, or while loitering, to allow the UAV to spend more time patrolling its target.
One prior art engine for a UAV is a diesel engine that drives a propeller. The diesel engine is a relatively high efficiency engine so the fuel consumption is very low. However, the diesel engine is a relatively heavy engine which must be carried by the aircraft, and thus less fuel and/or payload can be carried. Small gas turbine engines have been considered for use in a UAV but are not as efficient when compared to a diesel engine unless a recuperator is used. Adding a recuperator to a small gas turbine engine on a UAV creates a rather large engine. A rotary engine has also been used to power a UAV but is unreliable because these engines do not last very long. In some cases, the aircraft does not even make it back to the base and thus the entire aircraft is lost.
To be effective for use on a UAV, the engine must be able to fly at three speeds. The engine must have the capability of high enough power for takeoff. The engine must also have the power for what is referred to as dash speed when the aircraft is airborne and must fly to the destination rather quickly. Then, the most important operational speed for the engine is loiter or low speed which is when the aircraft must fly for long periods of time at the most fuel efficient rate. One major disadvantage of the gas turbine engine is that the engine is designed to operate at one speed with a high efficiency. At lower operational speeds, the gas turbine engine is at a relatively low efficiency.